1. Field of the Invention
This invention relates to a superposition processing method for a radiation image, wherein an addition process is carried out on a plurality of image signals, which represent a radiation image of a single object or radiation images of the single object. This invention also relates to an energy subtraction processing method, wherein a subtraction process is carried out on a plurality of image signals representing radiation images of a single object.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Further, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
Techniques for carrying out superposition processing on radiation images have heretofore been disclosed in, for example, U.S. Pat. No. 4,356,398. In general, radiation images are used for diagnoses of illnesses and for other purposes. When a radiation image is used for such purposes, it is required that even small differences in the radiation energy absorption characteristics among structures of an object can be detected accurately in the radiation image. The extent, to which such differences in the radiation energy absorption characteristics can be detected in a radiation image, is referred to as the contrast detection performance or simply as the detection performance. A radiation image having better detection performance has better image quality and can serve as a more effective tool in, particularly, the efficient and accurate diagnosis of an illness. Therefore, in order for the image quality to be improved, it is desirable that the detection performance of the radiation image may be enhanced. Practically, the detection performance is adversely affected by various noises.
For example, in radiation image recording and reproducing systems using stimulable phosphor sheets, it has been found that the noises described below occur during the step for recording a radiation image on a stimulable phosphor sheet and reading out the radiation image therefrom.
(1) A quantum noise of radiation produced by a radiation source. PA0 (2) A noise due to nonuniformity in how a stimulable phosphor coated on the stimulable phosphor sheet is distributed or how stimulable phosphor grains are distributed on the stimulable phosphor sheet. PA0 (3) A noise of stimulating rays, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during its exposure to radiation. PA0 (4) A noise of light, which is emitted by the stimulable phosphor sheet, guided and detected. PA0 (5) An electric noise in the system for amplifying and processing an electric signal.
Superposition processing is carried out in order to reduce the aforesaid noises markedly so that even small differences in the radiation energy absorption characteristics among structures of an object can be found accurately in a visible radiation image, which is reproduced finally, i.e. the detection performance of the radiation image can be improved markedly. Ordinary techniques and effects of the superposition processing are as described below.
A radiation image is stored on each of a plurality of recording media, which have been placed one upon another. Thereafter, an image read-out operation is carried out for each of the recording media. A plurality of image signals, which have been obtained from the image read-out operations, are then superposed one upon another. In this manner, various noises described above can be reduced. Specifically, in general, noises described in (1) through (5) for the stimulable phosphor sheets exhibit different distributions for different radiation images stored on the stimulable phosphor sheets. When the image signals detected from the stimulable phosphor sheets are superposed one upon another, the noises can be averaged. Therefore, the noises become imperceptible in a superposition image, which is obtained from superposition processing. Specifically, an image signal having a high signal-to-noise ratio (S/N ratio) is obtained from superposition processing. The same effects can be obtained also when radiation images having been recorded on sheets of X-ray film are read out. More specifically, most of the noises described in (1) through (5), particularly, the noise described in (1), which is one of dominant factors among the noises in a radiation image, can be approximated by the Poisson statistics. In cases where noises can be approximated by the Poisson statistics and two radiation images yield equivalent levels of signals S1 and S2 and equivalent levels of noises N1 and N2, the level of the signal corresponding to a superposition image, which is obtained by carrying out superposition processing on the two radiation images, becomes equal to S1+S2, and the level of noise in the superposition image is represented by Formula (1). ##EQU1## As for the signal-to-noise ratio, which is one of indexes representing the detection performance of a radiation image, the signal-to-noise ratios of the two radiation images prior to superposition processing are represented by the formulas S1/N1 and S2/N2. After superposition processing has been carried out on the two radiation images, the signal-to-noise ratio of the resulting superposition image is represented by Formula (2). ##EQU2## Therefore, as a result of superposition processing, the signal-to-noise ratio can be improved. When superposition processing is carried out on image signals representing the two radiation images, the values of the image signals may be weighted such that a markedly high signal-to-noise ratio can be obtained.
By way of example, when superposition processing is to be carried out by using the stimulable phosphor sheets, two stimulable phosphor sheets have heretofore been housed in a cassette such that they overlap one upon the other. Radiation images of an object are then recorded on the two stimulable phosphor sheets housed in the cassette. Thereafter, an image read-out operation is carried out on each of the two stimulable phosphor sheets, and two image signals are thereby obtained.
Also, techniques for carrying out subtraction processing on radiation images have heretofore been known. When subtraction processing is to be carried out, a plurality of (basically, two) radiation images recorded under different conditions are photoelectrically read out, and digital image signals which represent the radiation images are thereby obtained. The image signal components of the digital image signals, which represent corresponding picture elements in the radiation images, are then subtracted from each other, and a difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method, the plurality of digital image signals are subtracted from each other in order to obtain a difference signal, and the radiation image of a specific structure can be reproduced from the difference signal.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing method or the so-called energy subtraction processing method. In the former method, in order to extract the image of a specific structure of an object from the image of the entire object, the image signal representing a radiation image obtained without injection of contrast media is subtracted from the image signal representing a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media. In the latter method, an object is exposed to several kinds of radiation having different energy distributions. Alternatively, the energy distribution of the radiation carrying image information of an object, is changed after it has been irradiated onto one of a plurality of radiation image recording media, after which the radiation impinges upon the second radiation image recording medium. In this manner, a plurality of radiation images, in which different images of a specific structure of the object are embedded, are obtained. Thereafter, the image signals representing the plurality of radiation images are weighted appropriately, when necessary, and subjected to a subtraction process, and the image of the specific structure of the object is thereby extracted.
In the aforesaid radiation image recording and reproducing systems utilizing the stimulable phosphor sheets, the radiation image stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with such radiation image recording and reproducing systems, the aforesaid subtraction processing can be carried out easily. In cases where energy subtraction processing is to be carried out by using the stimulable phosphor sheets, radiation images may be stored on, for example, two stimulable phosphor sheets such that the parts of the radiation images corresponding to a specific structure may be different in the two radiation images. For this purposes, two-shot energy subtraction processing may be employed wherein the operation for recording a radiation image is carried out twice with two kinds of radiation having different energy distributions. Alternatively, one-shot energy subtraction processing may be employed wherein, for example, two stimulable phosphor sheets placed one upon the other (they may be in contact with each other or spaced away from each other) are simultaneously exposed to radiation, which has passed through an object, such that they may be exposed to radiation having different energy distributions.
As a method for photoelectrically detecting light emitted by a stimulable phosphor sheet, a method for detecting light emitted by two surfaces of a stimulable phosphor sheet has been proposed in, for example, U.S. Pat. No. 4,346,295. With the proposed method for detecting light emitted by two surfaces of a stimulable phosphor sheet, two photoelectric read-out means are located on opposite sides of the stimulable phosphor sheet. The two surfaces or only one surface of the stimulable phosphor sheet is scanned with the stimulating rays, and the light emitted by the two surfaces of the stimulable phosphor sheet is photoelectrically detected by the two photoelectric read-out means. With the proposed method for detecting light emitted by two surfaces of a stimulable phosphor sheet, a single radiation image is stored on the stimulable phosphor sheet, and the light emitted by two surfaces of the stimulable phosphor sheet is detected on the two sides of the stimulable phosphor sheet. Therefore, the efficiency, with which the light emitted by the stimulable phosphor sheet is guided and detected, can be kept high, and a high signal-to-noise ratio can be obtained.
With the method for detecting light emitted by two surfaces of a stimulable phosphor sheet, which has been proposed in U.S. Pat. No. 4,346,295, the stimulable phosphor sheet is placed on a transparent holder, and two photoelectric read-out means are respectively located above and below the holder. Specifically, the light emitted from the front surface of the stimulable phosphor sheet is detected by the photoelectric read-out means, which is located above the holder. Also, the light emitted from the back surface of the stimulable phosphor sheet is detected by the photoelectric read-out means, which is located below the holder.
When the image signals to be subjected to the superposition processing are to be obtained, for example, it is necessary to record radiation images on a plurality of stimulable phosphor sheets superposed one upon another. In such cases, the image signal, which is obtained from a stimulable phosphor sheet located at the position remote from the radiation source, contains image information in the low frequency band as in the image signal, which is obtained from a stimulable phosphor sheet located at the position close to the radiation source. However, in the image signal, which is obtained from the stimulable phosphor sheet located at the position remote from the radiation source, the frequency dependency in the high frequency band is lower than in the image signal, which is obtained from the stimulable phosphor sheet located at the position close to the radiation source. In the image signal, which is obtained from the stimulable phosphor sheet located at the position remote from the radiation source, as for the high frequency band, the amount of image information becomes small, and the amount of the noise components due to the effects of scattered radiation, or the like, becomes large. Therefore, if the image signal, which is obtained from the stimulable phosphor sheet located at the position remote from the radiation source, and the image signal, which is obtained from the stimulable phosphor sheet located at the position close to the radiation source, are weighted in the same manner and added to each other, the image quality can be kept good in the low frequency band in the addition signal obtained from the addition process, but the noise components will be emphasized and adversely affect the image quality in the high frequency band. Such adverse effects upon the image quality will also occur with the image signal, which is obtained from the front surface of the stimulable phosphor sheet by the method for detecting light emitted by two surfaces of the stimulable phosphor sheet, and the image signal, which is obtained from the back surface of the stimulable phosphor sheet. Further, as for the image signals to be subjected to the energy subtraction processing, the proportion of the noise components varies for different frequency bands of the image signal. Therefore, when the subtraction process is carried out on the image signals, it often occurs that the amount of the noise components in the difference signal becomes large, depending on the weight factor employed for each of the image signals.
Recently, it is desired to increase the speed, with which a radiation image is read out. Therefore, in Japanese Unexamined Patent Publication Nos. 60(1985)-117212 and 62(1987)-90615, the applicant proposed radiation image read-out apparatuses, which are capable of quickly reading out a radiation image from a stimulable phosphor sheet.
Also, in order for the speed, with which a radiation image is read out, to be increased, it has been proposed to increase the response speed of a stimulable phosphor with respect to stimulating rays, i.e. to increase the speed of light emission response of the stimulable phosphor with respect to the irradiation of the stimulating rays, with a method wherein, for example, cerium is added to the stimulable phosphor. When the response speed of the stimulable phosphor sheet with respect to the stimulating rays is kept higher, it becomes necessary to employ stimulating rays having a higher output power for the scanning of the stimulable phosphor sheet. Therefore, the stimulable phosphor sheet is quickly scanned with a laser beam, which serves as the stimulating rays and which has a high output power of at least 50 mW.
However, in cases where the aforesaid operation for quickly reading out a radiation image is carried out, the stimulating rays are moved very quickly on the stimulable phosphor sheet. Therefore, with certain kinds of stimulable phosphors constituting the stimulable phosphor sheets, the problems often occur in that the stimulable phosphor sheet cannot emit light immediately after being exposed to the stimulating rays, and a time lag occurs between when the stimulable phosphor sheet is exposed to the stimulating rays and when the stimulable phosphor sheet emits light. At an image contour portion, or the like, the amount of the light emitted by the stimulable phosphor sheet changes sharply. Therefore, the values of the image signal, which correspond to an image contour portion, or the like, should change sharply in the main scanning direction. However, if the time lag occurs between when the stimulable phosphor sheet is exposed to the stimulating rays and when the stimulable phosphor sheet emits light, the values of the obtained image signal, which correspond to an image contour portion, or the like, will not change sharply in the main scanning direction. As a result, the visible image reproduced from the image signal becomes unsharp in the main scanning direction, and the sharpness of the reproduced image cannot be kept high.
As for an image signal actually obtained from a stimulable phosphor sheet, or the like, the response characteristics will vary for different frequency bands. Therefore, such that a well-balanced visible image can be reproduced, it is desired to enhance the signal-to-noise ratio by reducing the noise components in the obtained addition signal or the obtained subtraction signal, and to emphasize or reduce the components of a desired frequency band in the obtained addition signal or the obtained subtraction signal. In such cases, it is necessary to carry out filtering processing on the obtained addition signal or the obtained subtraction signal by using, for example, a mask filter shown in FIG. 35. However, in such cases, the problems occur in that a long calculation time is required to carry out the filtering processing, and in that a complicated apparatus must be used.
Noise contained in an image signal is also affected by the dose of radiation delivered to the stimulable phosphor sheet. Specifically, when the dose of radiation delivered to the stimulable phosphor sheet is large, the proportion of the fixed noise due to the structure of the stimulable phosphor sheet, such as the state in which the stimulable phosphor is applied to the stimulable phosphor sheet, becomes higher than the proportion of the quantum noise of the radiation. Therefore, the ratio, in which a frequency band of the image signal is to be weighted such that the image quality of the reproduced image obtained from the addition signal or the subtraction signal may be kept best, varies for different doses of radiation delivered to the object.